Uninterruptible power supplies (UPS's) have long been used to provide at least temporary auxiliary power to electronic devices. Typically, a UPS is configured to switch between a primary power source and a standby power source as necessary to maintain constant power to a load. Typically, the primary power source for a UPS is a utility power supply, and the standby power source may take the form of a battery system. The UPS will normally operate in a line mode in which the utility power signal is passed to the load when the utility power signal is within predefined parameters. In the line mode, the UPS will typically also charge the battery system. When the utility power falls outside of the predefined parameters, the UPS will switch to standby mode in which an AC signal is generated based on the energy stored in the battery system.
A class of UPS's employs a ferroresonant transformer. A ferroresonant transformer is a saturating transformer that employs a tank circuit comprised of a resonant winding and capacitor to produce a nearly constant average output even if the input to the transformer varies. A typical UPS employing a ferroresonant transformer takes advantage of the voltage regulating properties of a ferroresonant transformer in both line and standby modes. In the context of a UPS, a ferroresonant transformer thus provides surge suppression, isolation, short circuit protection, and voltage regulation without the use of active components.
Conventionally, in line mode, a UPS employs an inverter circuit configured to form a switch mode power supply. An inverter circuit configured as a switch mode power supply typically comprises at least one and typically a plurality of power switches that are operated according to a pulse-width modulated (PWM) signal. The PWM method of generating an AC signal from a DC source allows the amplitude of the AC signal to be determined at any point in time by controlling the duty cycle at which the inverter power switches are operated. Controlling the duty cycle at which the inverter power switches are operated produces, through an output LC filter, a desired net average voltage. Typically, the parameters of the inverter control signal are varied according to a control signal generated by a feedback loop having an input formed by at least one characteristic, such as voltage, of the AC signal.
In a switch mode power supply, one of the major causes of loss of efficiency arises from the imperfect switching characteristics of modern power switches during the transition between the ON and OFF configurations of the power switches. An object of the present invention is to provide switch mode power supplies for use in UPS systems having improved efficiency.